The present invention relates to an improved web transport arrangement for synchronizing the feed rate of a web into a processing area with the rate of processing by said processing mechanism.
The need for an accurate automatically controlled web transport system becomes especially significant in the production of long lengths of certain web materials. One exemplary type of operation in which such an accurate control becomes highly desirable is in a rotary web-fed business forms printing press where a web which has been previously printed or processed is to be reprinted or reprocessed in order to perform second (or successive) operations in mutual registry with the results of the previous operation. Another exemplary type of operation in which it is beneficial to utilize an accurate control system is in the control of displacement feed of a web to effect metering over long lengths to assure accurate sheet lengths without the need for manual adjustment, regardless of the caliper of the web and without the undersirable effects of varying web tension within the processing mechanism.
In the production of such long lengths of web materials, even if only a slight difference exists between the operational rate of the processing apparatus and the rate of feed of the web, this difference over a great number of repeated operations can become relatively significant. Such a difference can result, for example, from a variation in the caliper of the web, or variations in the calipers of various webs utilized. If, for example, the web material is fed at too slow a rate, then the web material may be excessively stretched in the processing area. On the other hand, if the web material is fed at too fast a rate, then the tension of the processing area will not be great enough for proper registration between the processing equipment and the web material.
When a long length of web material, such as for the production of several hundred or thousand sheets, is produced in this manner, a minute difference between the feed rate of the web and the movement of the processing equipment can accumulate into a significant error if not corrected. A difference of a 100th of an inch, for example, will eventually mount up so as to result in the printed material being printed on the wrong section of the web. If this occurs, then when the web material is later cut into equally sized sheets, a portion of the printed material will be cut off, for example, from the bottom of one sheet and will be present at the top of the next sheet. Accordingly, in order to overcome this problem of synchronization in the processing operation, accurate control of the feed rate of the web becomes necessary.
An additional problem for consideration, is to provide for such an accurate control of the web feed without the necessity of utilizing a plurality of feed holes in a feed hole strip along the side of the web material with a corresponding pin feed device. The utilization of feed holes strips along the sides of the web merely constitutes waste paper in the final product. Hence, it would be desirable to eliminate the necessity of having such feed hole strips along the sides of the webs. For this purpose, it is necessary to eliminate the necessity of utilizing a pin feed device to accurately control the movement of the web material.
A separate feed mechanism is generally provided for controlling the rate of movement of the web material along the feed path in a direction towards the processing mechanism. While the web as it passes through the processing area is generally maintained at a relatively high tension, the web as it passes through the feed section is preferably maintained at significantly lower tension. For this reason, a tension-isolating mechanism is provided for separating the feed and processing areas. A web transport arrangement of this type is described in my copending U.S. application Ser. No. 459,175, filed Apr. 8, 1974, now abandoned, which application was replaced by a continuation application, Ser. No. 598,288, filed July 23, 1975, and now U.S. Pat. No. 3,995,373 which is commonly owned herewith; the subject matter of said patent being incorporated herein by reference.
In one of my earlier developments, a compensated displacement drum is provided for metering, at low tension, a web of paper into the high tension processing area. The rate of displacement of this metering drum can be altered to maintain accurate sheet length displacement as the caliper of the web varies. In accordance with such development, compensation for any expected incremental displacement change of a particular web is manually accommodated by utilizing change gears for incrementally correcting the feed rate of the drum. This incremental variation is calculated and gears changed according to the caliper of the web being fed. While such a web metering arrangement makes it possible to establish a basic sheet length accuracy, the system is not self-correcting as paper caliper varies and hence the previously mentioned problems could still exist to a limited extent.
Furthermore, while the system of my prior development provides for a manual procedure for compensating for incremental changes in the feed rate of the web, it would be desirable to provide a feed system in which the web being fed can be automatically compensated for feeding displacement, without the necessity of attendance by an operator. Previously known mechanisms for accomplishing such an automatic compensation rely on the correction of the feed rate by varying either the infeed ratio or by varying the web tension at the metering mechanism, but they exhibit the undesirable characteristic of varying the web tension at the infeed metering mechanism and also effecting corresponding tension changes in successive stages of the processing mechanism. The effect of such variations in the tension within the processing mechanism causes variations in color-to-color or print-to-punch operations as the web elastically elongates or contracts under the varying tension.